In my prior U.S. Pat. No. 3,997,071, I have provided electrical transducers proportional to each of the primary angles involved in the operation of a backhoe and combined the signals generated by such transducers according to a trigonometric equation to provide a signal which indicates the effective depth of the teeth of the bucket of the backhoe relative to the pivotal mounting of the backhoe on a vehicle. In a modification described in the aforementioned patent, I have combined the depth indicating signal with a signal generated in accordance with the vertical displacement of the pivotal mounting of the backhoe relative to a reference plane defined by rotating a laser beam over the working area of the machine to provide a continuous indication of the vertical displacement of the digging teeth relative to the reference plane.
The aforedescribed apparatus continuously indicates to the operator the depth position of the digging teeth of the backhoe, but it is still left to the operator's skill to manipulate the various control levers which respectively control the three primary hydraulic cylinders involved in the operation of the backhoe, to cause the digging teeth to move in a path parallel to a desired reference plane and thus produce a trench bottom of not only the desired depth but also the proper slope.
As is well-known, the speed of the digging operation employing a backhoe is primarily determined by the rate of change of the angle between the outreach boom and the downreach boom of the backhoe. If the operator could concentrate on changing this angle during the digging stoke as rapidly as permitted by the soil conditions, without having to concurrently adjust the cylinders controlling the two other basic angles of the backhoe, the speed of the digging operation could be measurably increased, but no apparatus has heretofore been provided that would permit the operator to manually control only the hydraulic cylinder which determines the aforementioned angle and still maintain the digging teeth of the backhoe moving along a plane parallel to the desired slope of the bottom of the excavation.